There are three tools that I find valuable for one to become effective in using Twitter. This includes:
This allows you to manage multiple Twitter accounts. I also added my Facebook fan pages for various websites (rather than Facebook personal profile) for simultaneous update. You can schedule tweets at a later time too.
It has statistics that you can use to monitor which of the links you tweeted merited the most number of clicks.
It can also promote your latest blog post as you can add RSS feeds.
You can use this tool to monitor which of your tweets elicited a retweet and response from followers. You can post follow-up replies from their site too (example). I posted this at the sidebar of my blog with the intent of encouraging readers to check it out and join the discussion if interested without necessarily leaving this blog.
This is similar to MyBlogLog that you can add at your sidebar where you will be able to see members checking out your blog. This is a great tool if you want to connect with other Twitter users who passed by. Effective too if you want to see how your Twitter profile is growing. (example)